Electrical contact brush



Patented Aug. 15, 1959 UNITED STATES PATENT OFFICE ELECTRICAL CONTACTBRUSH Earl L. McKinstry and Charles M. Roseman,

Cleveland, Ohio, assignors, by mesne assignments, to Union Carbide andCarbon Corporation, a corporation oi! New York Application January 6,1949, Serial No. 69,596

1 Claim. (Cl. 171-325) This invention relates to electrical contactbrushes for use in sliding contact with commutators or contact rings ofelectrical machinery.

Most such brushes are composed of carbon or graphite, often in admixturewith a metal, usually copper. They are held in contact by a holder andspring which permit some motion of the brush to accommodate unevennessesin commutator or ring surface and to maintain constant contact as thebrush is shortened by wear. To conduct the electrical current to or fromthe brush, a flexible wire or cable shunt is fastened to an end of thebrush either by a rivet or bolt or by inserting an end of the shunt intoa hole in the brush and tamping with metal powder or amalgam. Thisinvention relates more particularly to tamped shunts.

The copper wire used in shunts is conventionally in the soft, fullyannealed state. It has been observed that with conditions of extremevibration of the brush and correspondingly rapid flexing of the shunt,mechanical failure almost always occurs first in the shunt. Forinstance, in controlled bench tests to failure of a large number of suchbrushes with shunts, 95% of the failures occurred in the shunt, at anaverage life of 1.6 millions of vibrations. This observation suggeststhe substitution of a stiffer shunt, for instance of work-hardenedcopper, and a similar test of brushes having full-hard copper shuntsshowed indeed that about 80% of the failures occurred by failure of thebrush or amalgam; but no great advantage was thereby gained because thefailures occurred at an average life of 1.5 millions of vibrations.

Prolonged vibration increases the electrical resistance of a tampedshunt connection many fold, long before failure of the brush or shunt.

The foregoing experimental observations suggest that real improvementmight be obtained by using a hard copper shunt and strengthening thebrush in conventional fashion by impregnating it with a synthetic resin.Attempts to achieve improvement in this way were disappointing, becauseincreases in brush material strength only do not affect the mechanicaland electrical properties of the tamped metal connection. Solutions ofthe usual phenol-formaldehyde, polyvinyl and ester resins applied to thefinished brush connection, to try to obtain mechanical and chemicalprotection of the tamped metal and the immediately surrounding brushmaterial, were unsuccessful because extreme dilution of the resins wasrequired to obtain practical penetration into the connection.

We have now discovered that the improvement sought is obtained whenfurfuryl alcohol containing a polymerization catalyst is used as animpregnant for the tamping material and surrounding brush stock. Afterthe brush cable connection is formed the resin-producing liquid isapplied locally, as with a dropper. This mobile, self-hardening,undiluted resin-producing liquid, readily penetrates the brush and avery materially more durable product is obtained. Thus, in testsduplicating those mentionedabove, a large group of brush-shuntcombinations embodying the features of hard drawn copper cable andfurfuryl alcohol resin impregnated, tamped amalgam connections were sodurable that 73% of them lasted 4.2 millions of vibrations beforefailure, these failures being in the copper shunt; only 27% of thefailures took place in the brush or amalgam, and these failures occurredat an average life of 1.5 millions of vibrations. Furthermore, after onemillion vibrations, the resistance had increased, on the average, only56%.

The accompanying drawing shows a partially cutaway view of a commutatormade in accordance with the present invention.

The invention is a brush composed of graphite or carbon, or mixturesthereof with a metal such as copper, provided with a hard-drawn coppershunt secured by tamped metal powder or amalgam to an end of said brush,the tamping material, adjacent cable end and immediately surroundingbrush material being impregnated with polymerized furfuryl alcohol. Theamount of impregnant is not critical; but enough should be used tosaturate the region in and about the shunt.

The degree of hardness of the copper shunt wire may be from one-quarterhard (20% cold reduction in area) to extra hard (75% cold reduction inarea), hard (60% cold reduction in area) being preferred. Partiallyannealed drawn copper with residual hardness within this range is alsosatisfactory.

We have observed that one result of the use of a hard shunt wire is todecrease the amplitude of vibration whip of the shunt in service, andthus to reduce local strain concentrations. Accordingly, it is expectedthat in the course of several years of service experience with theinvention it will be found that the service life will be even greaterthan indicated by the vibration tests mentioned hereinabove.

Other resin-producing liquids may be substituted for furfuryl alcohol,provided they are mobile, completely penetrate the tamping ma- 3 teriaiand surrounding brush material, deposit a high percentage of resin inthe finished connection, and will prevent a greater than two foldelectrical resistance rise in the connection when it is heatedcontinuously at 100 C. for 16 days.

We claim:

An electrical contact brush comprising a body and a shunt cable securedby tamped metal in a hole in said b dy; said body being composed oimaterial selected from the group consisting of carbon, graphite andmixtures of metal with graphite; the tamped metal, adjacent cable endand immediately surrounding brush material being impregnated withpolymerized furfuryl alcohol; and said shunt cable being composed of 15,174,887

copper having a hardness within a range of one quarter hard to extrahard.

EARL L. McKINS'I'RY. CHARLES M. ROSEMAN.

REFERENCES CITED The following references are of record in the tile 01this patent:

UNITED STATES PATENTS Number Name Date 1,485,942 Adams Mar. 4, 19242,066,176 Gix'vin Dec. 29, 1936 2,174,886 Kieter Oct. 3, 1939 KieferOct. 3, 1939

